1. Field of the Invention
The present invention relates to a liquid crystal display device and a control driver for a liquid crystal display device.
2. Description of Related Art
A flat panel display has been widely spreading as a man-machine interface. Since being especially superior to other flat panel displays (for example, a plasma display panel) in terms of a manufacturing technology, a yield, and a cost, a liquid crystal display device is applied to various fields.
A liquid crystal panel provided with the liquid crystal display device has a characteristic called V-T (Voltage-Transmittance) characteristic. Liquid crystal molecules of a pixel in the liquid crystal panel respond to a voltage of a certain level or more and an orientation of the liquid crystal molecules changes. The V-T characteristic indicates a relationship between a voltage for changing the orientation of the liquid crystal molecules and an amount of light passing through the pixel according to the voltage. The liquid crystal panel has the V-T characteristic that is unique for each panel and is nonlinear. For this reason, an applied voltage to the liquid crystal panel is determined by a control driver having a D/A converter which generates a nonlinear drive voltage with respect to a value of an input grayscale data based on the unique V-T characteristic for the liquid crystal panel in a common liquid crystal display device. Since an input image data supplied from outside to the liquid crystal display device is often a data of a gamma value (γ=2.2) corresponding to the CRT (Cathode-Ray Tube), the D/A converter included in the control driver is generally set so as to have a display characteristic of the γ=2.2 for example.
In a typical liquid crystal display device, processing (hereinafter, referred to as gamma correction processing) for using respectively different gamma values for R(red), G(green), and B(blue) is sometimes performed in order to further improve a color tone of a display image. The typical liquid crystal display device includes a LUT (Look-up table: reference table) storing gamma characteristic (grayscale correction characteristic) data in a preceding step of the control driver in order to perform the gamma correction processing, and transfers image data converted from input image data by using the LUT to the control driver.
When the input image data is formed of 8 bits for example, the LUT of the liquid crystal display device is required to have the extended bit number such as 10 bits. This is required for preventing destruction of the data when the gamma correction processing is performed with referring to the LUT. Accordingly, in the typical liquid crystal display device, the LUT is included in a memory able to store data whose bit number is larger than the bit number of the input image data.
In this liquid crystal display device, techniques for suppressing increase of a memory capacity allocated to the LUT are known. For example, Japanese Laid-Open Patent Application JP-A-Heisei 5-64110 discloses a following technique. A display screen is divided into blocks and gamma correction data for every some blocks are stored in a plurality of LUTs. Image signals converted into digital signals by an A/D converter are input to the plurality of the LUTs, and image signals of the blocks whose gamma correction data doesn't exist are formed by an interpolation processing circuit including a coefficient addition circuit and an addition circuit.
In addition, for another example, Japanese Laid-Open Patent Application JP-P 2001-238227A (corresponding to U.S. Pat. No. 6,795,063B2) discloses a following technique. When an image is displayed by using elements having a nonlinear signal-brightness characteristic such as the liquid crystal display device, in adjusting the gamma characteristic and a white balance, a dynamic range corrected by digital data is set through a gain adjustment and an offset adjustment using an analog circuit. Accordingly, when correction by using the digital data is performed based on the Look-up table, increase of capacity of a memory used for data for correction can be suppressed by efficiently using all of the correction data.
In addition, for another example, Japanese Laid-Open Patent Application JP-P 2005-135157A (corresponding to US US2005111046 A1) discloses a following technique. An image processing circuit, an image display device and an image processing method for a grayscale correction can reduce a storage capacity of correction characteristic data. In the technique disclosed in JP-P 2005-135157A, grayscale correction characteristic data corresponding to the number of grayscales less than the number of grayscales of input image data is stored in first and second LUT storage sections. With reference to the first and second LUT storage sections, using a grayscale value of a pixel targeted for the grayscale correction processing as an input grayscale value, an output grayscale value corresponding to the input grayscale value and an output grayscale value corresponding to the input grayscale value adjoining it are obtained. The adjoining grayscale value means a grayscale value in a next upper level of a certain input grayscale value or a grayscale value in a next lower level of the certain input grayscale value. Then, an output grayscale value between the adjoining two output grayscale values is calculated by a linear interpolation and output grayscale values corresponding to all input grayscale values are obtained. Finally, a grayscale correction is performed for each pixel of input image data and corrected image data is output.
We have now discovered a following fact. In a liquid crystal display device, when gamma operation processing is performed, a gamma value of changed data is sometimes required to be changed depending on a contrast of displayed image and brightness around a display device. For this reason, it is required that a data conversion can be performed based on a plurality of the gamma values in the gamma operation processing for the input image data. Accordingly, when there is a plurality of the gamma values targeted to be changed, the same number of the LUTs as the number of the gamma values targeted to be changed are required to be installed. In order to install a plurality of the LUTs, a memory capacity able to store a plurality of the LUTs is required. When the plurality of the LUTs for performing the gamma correction processing is installed in a control driver, a problem of increasing a chip size occurs.
In addition, to realize the data conversion using a plurality of the gamma values targeted to be changed with suppressing an increase of the chip size in the control driver, it is required to employ one LUT to be included and to rewrite the LUT depending on a change of a gamma value of a displayed image. However, the rewriting of the LUT needs much time. For this reason, it is sometimes hard to rewrite the LUT in real time based on a change of an environment where an electric device is used.
In addition, a typical LUT can be also applied to correction processing of the V-T characteristic (hereinafter, referred to as a V-T correction processing) determined by a control driver generating a nonlinear drive voltage.
However, since the gamma correction processing or the V-T correction processing by the LUT is configured with the larger bit number than the bit number of the input image data in order to prevent a destruction of data, subtractive color processing is required to be performed before converted image data is input to the control driver when the correction processing by the LUT is performed.
Furthermore, in above mentioned correction processing employing one LUT according to the typical liquid crystal display device, it is impossible to simultaneously perform the processing for converting input image data into image data suitable for the liquid crystal display device such as the gamma operation processing (or other image calculation processing) and the V-T correction processing for further adapting the converted image data to the respective V-T characteristics of the display panel. In addition, the typical control driver cannot provide data to the display panel without performing the subtractive color processing.